In the construction of any battery, six elements must be present, namely, a positive and negative electrode, a housing, a separator, an electrolyte, and current collectors. These elements are incorporated into lithium ion batteries in thin sheets. The joining of the electrodes with the separator in lithium ion batteries is an important part of the manufacturing process. First, the bond between separator and electrode must be strong enough to prevent separation of the electrodes from the separator. Such separation impairs battery function. Second, the bond must provide for a high degree of contact between the separator and electrode so as to permit efficient ion transfer. Third, the distance between electrodes must be as short as possible to permit efficient ion transfer, while still providing a restriction to electron flow that would cause a short circuit.
The anode of a lithium ion battery may be constructed from a carbonaceous material suspended in a polymer matrix. These carbonaceous materials can reversibly accept and donate significant amounts of lithium. Examples of suitable carbonaceous materials include synthetic and natural graphite, petroleum coke and doped coke. The anode may be constructed from transition metal compounds having layered structures into which lithium ions can be intercalated and deintercalated during charge and discharge. These cells are referred to as rocking chair cells. The rocking chair compounds have electrochemical potentials close to that of lithiated carbon and different from the transition metal oxides frequently used for cathodes. Examples of suitable rocking chair anode compounds are Li.sub.x WO.sub.2, Li.sub.x MoO.sub.2 and Li.sub.x TiS.sub.2.
Transition metal oxides are preferred for lithium ion battery cathodes. Li ions can be inserted into and extracted from these intercalating compounds with little or no structural modification of the compound. Examples of suitable transition metal oxides for cathode construction include LiCoO.sub.2, LiNiO.sub.2 and LiMn.sub.2 O.sub.4.
Anode and cathode active materials are often formed into anode and cathode structures by binding the active materials in a polymer film or sheet. U.S. Pat. Nos. 4,980,250, 5,219,680, 5,340,670, 5,380,606, 5,426,006, 5,582,931, 5,584,893, 5,643,695 and 5,656,393, incorporated herein by reference, disclose compositions for Li ion cathodes and anodes and methods of making these electrodes. Once the polymer films containing the electrode active materials have set, the electrodes, separator and collectors are generally laminated by heat and pressure.
The separator must provide sufficient insulation between electrodes to prevent the formation of an electrical circuit which causes a short, while at the same time being permeable to migrating lithium ions. Separators have been constructed of very thin sheets of polymer, which are rendered porous by removal of plasticizers from cast films. The polymer matrix may be composed of olefin, polyvinyl alcohol, polyvinylidene difluoride and associated copolymers and the like. Alternatively, a preformed porous woven or nonwoven polymer mat may be utilized as a separator.
The electrolyte of lithium ion batteries consists of a lithium salt in a nonaqueous solvent. Typical lithium salts include LiPF.sub.6, LiASF.sub.6, LiBF.sub.4, LiClO.sub.4, LiN(CF.sub.3 SO.sub.2).sub.3 and LiN(SO.sub.2 C.sub.2 F.sub.5).sub.3. A nonaqueous environment is maintained in Li ion batteries because lithium and its salts are notoriously reactive in aqueous solutions. Aprotic organic solvents such as propylene carbonate or ethylene carbonate are commonly used. Lithium salts readily disperse in these solvents. Other solvents are tetrahydrofuran, 1,2-dimethoxyethane, dimethyl carbonate, diethyl carbonate, and diethoxyethane. The polymer separator, of course, must be stable to the solvent selected. For a discussion of conventional solvent/lithium solute systems, see S. Hossain, "Rechargeable Lithium Batteries (Ambient Temperature)", in Handbook of Batteries and Fuel Cells, D. Linden, Ed., McGraw-Hill, 2nd Ed., 1995, incorporated herein by reference.
The components of lithium ion batteries have been joined by heat and pressure lamination, screen printing, or a combination of the two. U.S. Pat. Nos. 5,584,893, 5,266,299, 5,425,932, 5,540,741, 5,470,357, 5,552,239, 5,219,680, 5,643,695, 5,426,006, 5,656,393 and 5,456,000 disclose methods of forming anode and cathode films from polymer slurries and then laminating these films to a separator and collectors. Such construction methods have certain disadvantages. Joining the components by heat and/or pressure results in a relatively weak bond. A laminated battery may delaminate due to forces generated during charge and discharge, causing battery failure. Second, large scale manufacture of individual polymer films is difficult because sheets of these materials are fragile and subject to fracturing, shearing and other degradation. Third, manufacture of separate anode, cathode and separator polymer films and subsequent joining by lamination consists of many separate manufacturing steps.
U.S. Pat. No. 5,582,931 (Kawakami, et al. 1996) represents an attempt to solve some of these manufacturing problems. The inventors screen print anode and cathode slurries onto a separator and allow the slurries to air dry. Conductors, and where appropriate, a lithium metal negative electrode are then heat and pressure connected to the positive electrode and separator. The cast electrodes are allowed to air dry, which occurs as solvent evaporates from the slurry into the atmosphere. However, the electrodes formed by this process may be compromised. Electrode slurries contract as they dry. When electrode slurries are simply allowed to air dry, the outside surface of the electrode (the side not contacting the separator) dries first, forming a "skin" around the undried inner portion of the electrode which still contains unevaporated solvent. As the inner portion of the electrode continues to dry, stress develops because of constraints imposed on the continued contraction of the inner electrode by the dry outer "skin." This drying/curling stress can cause debonding of the electrode from the separator, leading to poor battery performance.